1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a spherical semiconductor device comprising a spherical semiconductor element having one or more electrodes on its surface. The present invention relates also to a method for fabricating a semiconductor device and, more particularly, to a method for fabricating a spherical semiconductor device comprising a spherical semiconductor element having one or more electrodes on its surface.
2. Description of the Related Art
Recently, instead of conventional semiconductor devices fabricated by forming integrated circuits on silicon wafers, a spherical semiconductor element fabricated by forming an electric circuit on the surface of spherical silicon has been developed. This spherical semiconductor element has one or more electrodes on its surface. A semiconductor device having a variety of functions can be realized by combining spherical semiconductor elements having various functions.
Such a spherical semiconductor element cannot operate only by itself. It requires input/output means for electrical connection to the outside to exchange electrical signals with an external circuit or the like. Although spherical semiconductor elements have excellent functions, effective measures have not been found particularly for packaging.
It is an object of the present invention to provide a spherical semiconductor device having improved connectivity to the outside.
It is another object of the present invention to provide a method for fabricating a spherical semiconductor device having improved connectivity to the outside.
According to the present invention, a spherical semiconductor device comprises a spherical semiconductor element comprising one or more electrodes on a surface of the element and spherical conductive bumps formed at the positions of the electrodes.
According to an aspect of the present invention, said electrodes are arranged so as to contact a common plane.
According to another aspect of the present invention, the spherical bumps constituting a group to be connected to the outside, protrude above the spherical semiconductor element such that there is formed no gap or a predetermined gap between a plane or a spherical surface capable of contacting the group of spherical bumps, and the surface of the spherical semiconductor element.
According to another aspect of the present invention, each spherical bump is made of a refractory metal having a melting point of not less than 550xc2x0 C.
According to another aspect of the present invention, each electrode is made of a material selected from the group consisting of aluminum, copper, and an alloy containing at least one of aluminum and copper, and each spherical bump is made of a material selected from the group consisting of gold, platinum, palladium, silver, copper, aluminum, nickel, and an alloy containing at least one of gold, platinum, palladium, silver, copper, aluminum, and nickel.
According to another aspect of the present invention, each spherical bump is made of a low-melting metal having a melting point of not more than 450xc2x0 C.
According to another aspect of the present invention, each electrode is made of a material selected from the group consisting of aluminum, copper, and an alloy containing at least one of aluminum and copper, and each spherical bump is made of a material selected from the group consisting of lead, tin, indium, bismuth, zinc, an alloy containing at least one of lead, tin, indium, bismuth, and zinc, and an alloy mainly containing one of gold-silicon alloy, gold-tin alloy, and silver-tin alloy.
According to another aspect of the present invention, at least one metal layer selected from the group consisting of titanium, tungsten, titanium-tungsten, nickel, chromium, gold, palladium, copper, and platinum is formed on each electrode.
According to another aspect of the present invention, each electrode is connected through the spherical bump formed thereon, to an electrode of a ceramics substrate, a film carrier, a silicon substrate, a printed circuit board, a lead frame, a semiconductor chip, or a spherical semiconductor element.
According to another aspect of the present invention, each spherical bump is made of a refractory metal and connected through a low-melting metal to an electrode of a ceramics substrate, a film carrier, a silicon substrate, a printed circuit board, a lead frame, a semiconductor chip, or a spherical semiconductor element, and the difference in melting point between the refractory metal and the low-melting metal is not less than 50xc2x0 C.
According to another aspect of the present invention, each spherical semiconductor element is encapsulated with an encapsulating material.
According to another aspect of the present invention, each electrode has a shape selected from the group of a trapezoid, a polygon having at least five sides, and a circle.
According to another aspect of the present invention, each electrode has an area equivalent to the area of a circle having a diameter not less than 3% of a diameter of the spherical semiconductor element.
According to another aspect of the present invention, each spherical bump is made of a refractory metal coated with a low-melting metal.
According to the present invention, since spherical conductive bumps are formed at the positions of electrodes of a spherical semiconductor element, electrical connections to the outside can be easily and accurately made through the spherical bumps.
In particular, a group of spherical bumps to be connected to the outside are arranged to protrude above the spherical semiconductor element such that a predetermined gap is formed between a plane or a spherical surface capable of contacting the group of spherical bumps and the surface of the spherical semiconductor element. Since the spherical bumps thus protrude above the spherical semiconductor element, extremely superior bump joining properties can be obtained.
In case of melt joining, it can be performed by the wet effect of each bump melted even if there is formed no gap.
The surface of each spherical bump made of a refractory metal is coated with a low-melting metal. By setting the difference in melting point between the refractory and low-melting metals to 50xc2x0 C. or more, preferably, 100xc2x0 C. or more, the surface portion can be melted while the core remains solid during joining. So, a certain distance, i.e., a distance not less than the diameter of the core metal can be kept between the junction portions.
Each spherical bump may deform into a shape like a Rugby ball, or unevenly deform at its part, e.g., its junction portion. In order for the spherical bumps surely to protrude beyond an apex of the spherical semiconductor element, two or more layers of bumps may be used.
According to the present invention, since spherical conductive bumps are formed at the positions of electrodes of a spherical semiconductor element, electrical connections to the outside can be easily and accurately made through the spherical bumps. In this case, by arranging the spherical bumps to protrude above the spherical semiconductor element, extremely superior bump joining properties can be obtained. As a result, high reliability can be obtained when a semiconductor device comprising such a spherical semiconductor element is packaged or the like.
According to another aspect of the present invention, a method for fabricating a spherical semiconductor device having spherical bumps on surface electrodes of a spherical semiconductor element, comprises the steps of temporarily arranging conductive balls for forming the spherical bumps, on an arrangement substrate at positions respectively corresponding to said surface electrodes, and transferring the conductive balls onto the surface electrodes to join.
According to another aspect of the present invention, the conductive balls are transferred from the arrangement substrate to the surface electrodes while the position of each of the conductive balls on the arrangement substrate is regulated.
According to another aspect of the present invention, the conductive balls are transferred from the arrangement substrate to the surface electrodes such that a predetermined gap is formed between a surface of the arrangement substrate and a surface of the spherical semiconductor element.
According to another aspect of the present invention, the conductive balls are transferred onto and joined to the surface electrodes by thermo-compression bonding.
According to another aspect of the present invention, the conductive balls are transferred onto and joined to the surface electrodes by melting.
According to another aspect of the present invention, each conductive ball is transferred onto and joined to the corresponding surface electrode after one of the surface electrode and conductive ball is coated with a flux.
According to another aspect of the present invention, conductive balls are arranged on the arrangement substrate to correspond to electrodes of spherical semiconductor elements, and the conductive balls are transferred onto the spherical semiconductor elements at once from the arrangement substrate to form bumps.
The fabrication method according to the present invention uses an arrangement substrate having arrangement holes corresponding to surface electrodes of a spherical semiconductor element. Conductive balls are temporarily arranged on the arrangement substrate and then transferred onto the surface electrodes of the spherical semiconductor element, and thereby the conductive balls and the surface electrodes are brought into contact with each other while they are aligned with each other.
In this case, since the surface of the semiconductor element is spherical, the position of each conductive ball may deviate during the transfer process if it is simply placed on the arrangement substrate for temporary arrangement. In the present invention, therefore, positional regulation is effected when each conductive ball on the arrangement substrate is brought into contact with a corresponding electrode. This affords a proper and reliable transfer operation for the conductive balls.
According to the present invention, in fabricating a semiconductor device comprising such a spherical semiconductor element, conductive balls are temporarily arranged on an arrangement substrate and then transferred onto the surface electrodes of the spherical semiconductor element, and thereby the conductive balls and the surface electrodes are brought into contact with each other while they are aligned with each other. It is, therefore, possible to form spherical bumps of the conductive balls and having excellent characteristics, and realize good electrical connections to an external circuit or the like through the spherical bumps.